Coating particles by the decomposition of metal carbonyl is currently conducted on a large scale using fluidized bed reactors. See, for example, Canadian Patent No. 836,483.
In short, a gaseous metal carbonyl, frequently nickel tetracarbonyl Ni(CO).sub.4, (and less frequently iron pentacarbonyl Fe(CO).sub.5 or cobalt tetracarbonyl Co.sub.2 (CO).sub.8 or tricarbonyl Co.sub.4 (CO).sub.12) is brought into intimate contact with particulate matter within the fluidized bed. Depending on the conditions including a temperature range of about 149.degree.-316.degree. C., the carbonyl decomposes into nickel metal (Ni) and carbon monoxide (CO) with the nickel metal depositing on the particulate material. Toward the higher temperature range, carbon monoxide tends to disproportionate into carbon dioxide (CO.sub.2) and elemental carbon (C) with the carbon thereby contaminating the product. Accordingly, the temperature of the fluidized bed reactor must be monitored.
A further limitation of the fluidized bed reactor is that the size of the particles to be coated are on the order of fifty (50) micrometers and larger. Oftentimes, however, there is a need to coat a particulate substrate having a size of about ten (10) micrometers or less. The fluidized bed reactor is not suitable to coat such small particles. Moreover, by employing a fluidized bed reactor, the decomposition zone and the heating zone occur in one vessel requiring a higher volume and pressure of gas to keep the material moving throughout the fluidized bed reactor and associated system. Heating is accomplished at the bottom and decomposition at the top of the fluid bed at the expense of some metal depositing on the heating surfaces, feed gas nozzles, etc. Complete decomposition to allow recycling of the off-gas for fluidization is a further problem.